1. Field of the Invention
The present invention relates to a color picture image forming apparatus, such as a digital color copying machine, printer or facsimile machine, which generates mesh dot halftone images by comparing image data to a matrix pattern of threshold values having a screen angle with the picture image signals.
2. Description of the Related Art
Image forming equipment, such as laser printers and ink jet printers, which can only express image density in two chromatic grades, i.e., black and white, or in a small number of grades, have employed halftone image generating processes such as the density pattern process or the dither process for the recording of halftone images.
As illustrated in FIG. 55(a), the density pattern process comprises the steps of subdividing a picture element G, which has been read from an original picture, into n.times.m minute picture elements g (there are 5.times.5 minute picture elements in the example of FIG. 55), allocating a threshold value corresponding to each minute picture element g to form a matrix of binary values, sequentially comparing the average image density of picture element G with each threshold value of the binary matrix, forming a mesh dot image MG by shading the minute picture elements g having a threshold value larger than the overall image density white, and by shading the minute picture elements g having a threshold value smaller than the overall image density black, and recording the resulting mesh dot image MG as a halftone image.
As illustrated in FIG. 55(b), the dither process comprises the steps of dividing an original picture element G into n.times.m density elements, allocating a threshold value to each n.times.m minute picture element g of picture element G, in the same manner as described in the density pattern process, sequentially comparing the density of each minute picture element g with the corresponding density element of the original picture element G, forming a mesh dot image MG by shading each minute picture element g having a threshold value larger than the corresponding density element of the original picture element G white, and by shading each minute picture element g having a threshold value smaller than the corresponding density element of the original picture element G black, and recording the mesh dot image MG as a halftone image.
In these cases, the number of threshold values comprising a 5.times.5 element threshold value matrix pattern corresponds to a total of only 25 halftone grades.
These processes have also been applied to the recording of halftone images in multi-color printing. For example, color image forming machines such as digital color copying machines, printers and facsimiles that are provided with developing units with toners comprising three colors, Y (Yellow), M (Magenta), and C (Cyan), or four colors, Y, M, C, and K (Black), reproduce color images by converting signals corresponding to the density of Y, M, C, and K of the image into binary data, comparing this data with corresponding threshold values comprising a threshold value matrix pattern, and transferring the overlapping mesh dot images (i.e. dot matrices). However, the number of chromatic grades and the degree of resolution of such color image forming machines are inversely related. Therefore, an attempt to enlarge the matrix size in order to increase the number of chromatic grades will result in a deterioration of the resolution. Likewise, in order to increase the resolution, it is necessary to decrease the number of chromatic grades. Therefore, in order to achieve a large number of chromatic grades with high resolution, it is necessary to divide each single picture element comprising the matrix into sub-elements. Known methods for achieving this subdivision in laser printers include brilliance modulation and pulse width modulation. Brilliance modulation involves controlling the amount of light emitted by the laser beam while pulse modulation involves controlling the duration of time for which the laser beam remains turned on. Both of these methods form minute picture elements by the division of picture elements into smaller multiple-value parts in the scanning direction of the laser.
However, minute picture elements obtained by a subdivided dot picture element are often less stable than single-dot picture elements which are not subdivided. Therefore, it is desired to reproduce the dots by a process of fostering their growth with as much concentration as possible, as presented, for example, in Japanese Patent Application Unexamined Publication No. 214662-1986, or, more specifically, by a process of attaining the growth of dots by their straight linkage in the manner of a myriad line screen. In order to achieve image reproduction by this concentrated dot process, it is necessary to either (1) increase the resolution of the output of the records to a very high level or (2), as shown in FIGS. 56(a) through 56(c), develop a myriad line screen that simulates the arrangement of threshold values within the threshold value matrix utilized in the dither process. The process of FIGS. 56(a) through 56(c) comprises the steps of dividing one picture element into five minute picture elements, arranging them in the scanning direction of the laser, as illustrated in FIG. 56(a), and forming a threshold value matrix corresponding to the five minute picture elements.
Generally, when multi-color dot matrix halftone images are reproduced, a moire forms between the screens used for printing the different colors while printing the individually colored mesh dot images. In order to prevent the occurrence of a moire, the screens for printing the individual colors are set at different angles from one another. It is therefore necessary to change the contents of the threshold value matrices to reflect the different screen angles. The process by which the contents of the threshold matrix may be changed includes the method of preparing the basic threshold value matrix for each individual color and subsequently generating therefrom a threshold pattern corresponding to a selected screen angle, as described in Japanese Patent Application Unexamined Publication No. 85434-1983. However, a normal mesh dot pattern cannot be generated unless the basic threshold value matrix is in excess of a particular size.
To solve the foregoing problem, a threshold value matrix generating system in which the output dots are always grown from the center of the matrix, regardless of the size of threshold value matrix, has been proposed in Japanese Patent Application Unexamined Publication No. 149270-1987.
This threshold value matrix generating system is designed to generate a threshold value corresponding to a screen angle by specifying the line and row address of the basic threshold value block with respect to the screen angle, in reflection of the noted feature that the all the threshold value matrices for the image for one page as arranged with the prescribed angle set for the screen can be divided into basic threshold value blocks in a certain size. For example, the screen angle for the yellow output will be 18.5 degrees, the screen angle for the magenta output will be 45 degrees, the screen angle for the cyan output will be 71.5 degrees, and the screen angle for the black output will be 0 degree.
FIG. 57 presents an example arrangement of the threshold matrices where the screen angle is set at 18.5 degrees. In the figure, the block with hatching applied thereto represents the threshold matrix of FIG. 58(a). The basic threshold value block shown in FIG. 58(b) comprises two rows and twenty columns and is repeated a plurality of times in the main scanning direction as shown in FIG. 57. While the basic threshold value block is shifted by the prescribed amount in the main scanning direction, it is also repeated a plurality of times in the subsidiary scanning direction as shown in FIG. 59 such that the data initially read out of the basic threshold block is different for every two lines. FIG. 59 shows how the basic threshold value block of the threshold value arrangement shown in FIG. 57 is repeated wherein the number of lines K is 2, the number of rows L is 20, and the number of shifts S is 6. Moreover, the number of shifts from the initial basic threshold value block is 10 comprising S, 2S, 3S, 4S-L . . . , 9S-2L, and 0. With regard to the other colors having different screen angles, similar shift arrangements are used.
FIG. 60(a) and 60(b) show the threshold value matrix and the threshold value basic block, respectively, when the screen angle is 45 degrees. In this case, there are two kinds of shifts, O and S. FIGS. 61(a) and 61(b) show the threshold value matrix and the basic threshold value block, respectively, when the screen angle is 71.5 degrees. In this case, there are ten kinds of shifts each being identical to the shifts found when the screen angle is 18.5 degrees. Finally, FIG. 62 shows the case where the screen angle is 0 degrees, and wherein the threshold value matrix and the basic threshold value block are identical.
FIG. 63 is a schematic block diagram of the threshold value matrix generating system proposed earlier wherein the memory device 821 accommodates the threshold value data for the basic threshold value block. It provides the basic pattern and the size of the basic threshold value block in the main scanning direction and in the subsidiary scanning direction, depending on the screen angle, as mentioned earlier. Because the reading position for the memory device 821 must be changed in accordance with the main scanning position, the subsidiary scanning position, and the output colors, the system is designed such that it is possible to set, with the initial value setting device 824, the initial value of the main scanning counter device 822, in order to specify the address in the line direction of the memory device 821, and the initial value for the subsidiary scanning counter device 823, in order to specify the address of the memory device 821 in the row direction.
When performing the concentrated dot-type reproduction method, problems arise when trying to increase the degree of output resolution. For instance, in order to increase output resolution, the polygon mirror, which performs the scanning of the laser and the control of the video frequencies for controlling the on/off operations of the laser unit during the recording process, must be revolved at very high speeds, making the system difficult to control and impractical.
Moreover, during the process of developing a pseudo-myriad line screen with respect to the arrangement of threshold values in the threshold value matrix by the dither process, the threshold values are arranged in a state of dispersion such that the dots on the highlight side as shown in FIG. 56(b), i.e., the picture elements with small numerical values, are scattered. This can either result in an inferior reproduction of the original image or the development of a cyclic structure in the subsidiary scanning direction of the laser giving rise to a moire and a texture wherein a portion of the screen is disrupted in the direction of the subsidiary scanning, as shown in FIG. 56(c). The final result is an increase in noise making it impossible to obtain a high quality picture element. In order to avoid such problems, it is conceivable to arrange a threshold value to be connected to the subsidiary scanning direction. However, in this case, it is impossible to obtain a favorable result because of the considerable deterioration of the image reproduction due to the fact that it is not possible to set up a large number of chromatic grades.
Moreover, provided that the size of the basic threshold value block is K lines and L rows and that the number of bits necessary for representing the K lines is expressed by "k", the number of bits necessary for representing the L rows is expressed by "l", and the number of bits necessary for representing the matrix changeover signal SL is expressed by "m", the accesses necessary for the memory device 821 will comprise a total of l+k+m=n bits.
For example, assuming that L=20 and K=2, then "l=5 and k=1, which means that a total of six bits, five bits in the main scanning direction and one bit in the subsidiary scanning direction, will be necessary. On the other hand, if the basic threshold value block comprises six lines and six rows, then l=3 and k=3, which means that a total of six bits, three bits in the main scanning direction and three bits in the subsidiary scanning direction, will be necessary. Although in both examples, the total number of bits comprising the addresses for reading out the basic threshold value block is six, the number of bits comprising the main scanning direction and the subsidiary scanning direction are different. Therefore, it is necessary to provide independent address circuits for each example resulting in an overly complicated circuit. Furthermore, because only part of the memory area is used, there are problems of inferior memory-utilization rate and access time delays.